Wich hardware suits best for large full-text indexed databases

Hi folks,

I have a database using tsearch2 to index 300 000 documents.
I've already have optimized the queries, and the database is vacuumed on 
a daily basis.
The stat function tells me that my index has aprox. 460 000 unique words 
(I'm using stemmer and a nice stopword list).
The problem is performance, some queries take more than 10 seconds to 
execute, and I'm not sure if my bottleneck is memory or io.
The server is a Athlon XP 2000, HD ATA133, 1.5 GB RAM running postgresql 
7.4.3 over freebsd 5.0 with lots of shared buffers and sort_mem...

Does anyone has an idea of a more cost eficient solution?
How to get a better performance without having to invest some 
astronomicaly high amount of money?

TIA,

-- 
Diogo Biazus
diogo@ikono.com.br
http://www.ikono.com.br